1. Field of the Invention
The present invention is directed to digital projectors incorporating light engines with color splitting-converging prisms, and more particularly to a color splitting-converging prism incorporating a coating for reducing dichroic shift and thereby minimizing light loss.
2. Description of the Related Art
A typical color digital projector consists of a lamp, an illumination system, and a color splitting-recombining prism. Attached to the prism is an imaging device, typically a digital micromirror device (DMD) panel. The imaging device receives a light cone from the lamp and modulates the light to create an image on the screen.
The DMD is an electromechanical device consisting of millions of microscopic mirrors that modulates light by independently flipping each mirror through a +−12 degree angle. The design of the DMD requires creation of a light cone with an f/#2.4-2.5 for maximum light throughput in the projector. Using three such DMDs on a prism (e.g. Phillips® prism, plumbicon, etc.), a white light cone of f/2.4 can be separated into red, green, and blue. Each color is individually modulated by the DMD and then recombined by the prism.
The design of the plumbicon prism requires that the dichroic coatings be designed for large angles of incidence relative to normal (i.e. 90°). Typically, the dichroic coatings are designed for angles of 12-59 degrees from the normal incident ray. When light of f/2.4 strikes such a coating the dichroic shift is large above 20 degrees AOI. The dichroic shift limits the collection efficiency for light rays in the range of from approximately 30-59 degrees. This lowers the efficiency of light transmission in the prism and contributes to increased heating of the prism.
Accordingly, it is an objective of the present invention to provide a prism that reduces dichroic shift and thereby increases light transmission efficiency.